a drum driven by
clockwork.]

The first thing to be done by any port which wishes its tides to be
predicted is to set up a tide-gauge, or automatic recorder, and keep it
working for a year or two. The tide-gauge is easy enough to understand:
it marks the height of the tide at every instant by an irregular curved
line like a barometer chart (Fig. 117). These observational curves so
obtained have next to be fed into a fearfully complex machine, which it
would take a whole lecture to make even partially intelligible, but Fig.
118 shows its aspect. It consists of ten integrating machines in a row,
coupled up and working together. This is the "harmonic analyzer," and
the result of passing the curve through this machine is to give you all
the constituents of which it is built up, viz. the lunar tide, the solar
tide, and eight of the sub-tides or disturbances. These ten values are
then set off into a third machine, the tide-predicter proper. The
general mode of action of this machine is not difficult to understand.
It consists of a string wound over and under a set of pulleys, which are
each set on an excentric, so as to have an up-and-down motion. These
up-and-down motions are all different, and there are ten of these
movable pulleys, which by their respective excursions represent the
lunar tide, the solar tide, and the eight disturbances already analyzed
out of the tide-gauge curve by the harmonic analyzer. One end of the
string is fixed, the other carries a pencil which writes a trace on a
revolving drum of paper--a trace which represents the combined motion of
all the pulleys, and so predicts the exact height of the tide at the
place, at any future time you like. The machine can be turned quite
quickly, so that a year's tides can be run off with every detail in
about half-an-hour. This is the easiest part of the operation. Nothing
has to be done but to keep it supplied with paper and pencil, and turn a
handle as if it were a coffee-mill instead of a tide-mill. (Figs. 119
and 120.)

[Illustration: FIG. 118.--Harmonic analyzer; for analyzing out the
constituents from a set of observational curves.]

My subject is not half exhausted. I might go on to discuss the question
of tidal energy--whether it can be ever utilized for industrial
purposes; and also the very interesting question whence it comes. Tidal
energy is almost the only terrestrial form of energy that does not
directly or indirectly come from the sun. The energy